This invention relates to a system for controlling processing of an evaporated fuel occurring in a fuel tank in a vehicle such as an automobile, and more specifically, to an evaporated fuel control system using a learning control of a zero-point correction of a pressure sensor which detects an inner pressure in the fuel tank.
The control system generally prevents an air pollution caused by an emission of the evaporated fuel to an air in the manner that a canister once soaks the evaporated fuel occurring in an upper space of the fuel tank during stopping and running of the vehicle through an evaporation pipe, and the soaked fuel evaporated is purged through a purge pipe to an intake manifold to be burned while an engine is driven. Occurrence of the evaporated fuel changes in dependency on variable conditions such as an atmospheric temperature, an atmospheric pressure, a quantity and a temperature of fuel. And also an inner pressure in the fuel tank changes in dependency on conditions of the evaporated fuel at an initial and a purge states. Accordingly, an extremely high pressure occurs in the fuel tank and the evaporated fuel bursts out to the air at refueling when a large amount of the evaporated fuel occurs. But the fuel vapor can not fully purged while driving in a traffic congestion. In contrast, an over-negative pressure in the fuel tank causes the tank to be destroyed when a small amount of the evaporated fuel occurs and is continuously purged even though the fuel is cool.
Therefore, a pressure sensor is installed in the fuel tank to detect an inner pressure in order to prevent a trouble caused by the extremely-high or over-negative pressure in the fuel tank, thereby performing a reversion control to the inner pressure in the fuel tank to usually set to be the air pressure. An accuracy of the return-to-normal control by the pressure sensor is influenced by the changes of a sensor output due to an accuracy when produced and an aging of parts of the sensor. Accordingly, it is required that an output value from the sensor is corrected to be zero and further to perform a learning control causing a zero point to be proper in order to improve the detection accuracy of an internal pressure of the tank.
Hitherto, a prior art is disclosed in the official gazette of Japanese patent application laid-open No. 5-195896 (1993) with respect to a zero-point correction of the pressure sensor in the fuel tank. In this prior art, a second control valve of a purge pipe is closed and a first control valve in an evaporated pipe and a third control valve in an intake port of a canister are open when an engine is in condition of a cold start. At this time, internal pressure detection means detects an internal pressure value as a positive and negative pressure change point to be stored, and an output value of the internal pressure detection means is corrected in dependency on the positive and negative pressure change point.
Since the above prior art relates to a method in which the fuel tank is open to the air by closing the second control valve of the purge pipe and by opening the first control valve and the third control valve of the intake port of the canister, the internal pressure decreases step by step by soaking the evaporated fuel when the internal pressure in the fuel tank is high in dependency on an occurrence of the evaporated fuel. A time until the pressure in the fuel tank becomes an atmospheric pressure differs according to characteristics of the fuel. Furthermore, it is impossible to purge the evaporated fuel from the fuel tank during this time. Accordingly, it is difficult to cause the fuel tank to be usually the atmospheric pressure within a predetermined time, thereby disabling a proper zero-point correction. Furthermore, since the zero-point correction is performed at only a cold start of the engine, it is impossible to correspond the case where an output of the sensor changes after starting an engine.